Sensing rate of change of pressure in the left ventricle with an implanted device

ABSTRACT

An implantable device and method for monitoring S1 heart sounds with a remotely located accelerometer. The device includes a transducer that converts heart sounds into an electrical signal. A control circuit is coupled to the transducer. The control circuit is configured to receive the electrical signal, identify an S1 heart sound, and to convert the S1 heart sound into electrical information. The control circuit also generates morphological data from the electrical information. The morphological data relates to a hemodynamic metric, such as left ventricular contractility. A housing may enclose the control circuit. The housing defines a volume coextensive with an outer surface of the housing. The transducer is in or on the volume defined by the housing.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/178,945, filed Jul. 8, 2011, which is a continuation of U.S.application Ser. No. 12/703,533, filed Feb. 10, 2010, now issued as U.S.Pat. No. 8,007,442, which is a continuation of U.S. application Ser. No.11/142,851, filed Jun. 1, 2005, now issued as U.S. Pat. No. 7,670,298,which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

This document relates to cardiac rhythm management devices generally,and more particularly to cardiac rhythm management devices that employ asensing device to detect a heart sound and to extract morphological datatherefrom, in order to relate the heart sound to a hemodynamic metric.

BACKGROUND

Cardiac pacemakers generally provide functions including sensingelectrical signals generated by the heart, controlling stimulation ofexcitable tissues in the heart, sensing the response of the heart tosuch stimulation, and responding to inadequate or inappropriate stimulusor response (e.g., dysrhythmia) to deliver therapeutic stimuli to theheart. Some pacemakers employ cardiac resynchronization therapy. Someexisting cardiac pacemakers also function to communicate with anexternal programmer device to support a variety of monitoring,diagnostic and configuration functions.

Certain cardiac pacemakers, defibrillators with pacing and/or cardiacresynchronization therapy (CRT) capabilities, and CRT devices(collectively referred to herein by the term “pacemaker”) include aninternal accelerometer for measuring the level of activity of thepatient (e.g., movement caused by walking around, or by muscletwitches). Such pacemakers process (e.g., filter) the accelerometersignal to reduce noise interfering with the measurement of the patient'smotion-related activity, such as the sounds generated by the heartitself, and then use the processed signals as inputs to one or morealgorithms for generating the signals used to control the stimulation ofthe heart. For example, if the accelerometer indicates that a patient iswalking briskly, the pacemaker may stimulate the heart to beat at afaster rate (often subject to an upper rate limit) than when the patientis at rest.

Pacemakers are typically electrically coupled to a patient's heart by alead system. The lead system may include one or multiple leads that mayprovide electrical contact with one or multiple chamber of a patient'sheart. Some leads may contain an accelerometer at their distal end. Whenimplanted, the accelerometer is located within a patient's heart, andmay detect sounds emitted by the heart. Such a scheme may be used, forexample, to detect an S1 heart sound (an S1 heart sound is the firstsound made by the heart during a cardiac cycle). It is known that an S1heart sound contains data content related to left ventricularcontractility, a characteristic of the heart that reveals the capacityof the myocardium to shorten, and therefore to circulate blood throughthe body. A pacemaker system such as the one described may measure S1heart sounds as a means to gather information about the contractility ofthe patient's heart.

The above-described scheme exhibits certain shortcomings, however. Sucha scheme may lead to the use of two accelerometers—an internalaccelerometer for use in adjusting the pacing rate during instances ofphysical exertion by the patient, and an external accelerometer situatedin the heart for the purpose of monitoring heart sounds. Disposing anaccelerometer on the tip of a lead is costly, and could be avoided if aninternal accelerometer could be used to detect heart sounds with asufficient signal-to-noise ratio to permit extraction of data contentrelated to cardiac performance (such as left ventricular contractility).

SUMMARY

Against this backdrop the present invention was developed. According toone embodiment, an implantable device includes a transducer thatconverts heart sounds into an electrical signal. A control circuit iscoupled to the transducer. The control circuit is configured to receivethe electrical signal, identify an S1 heart sound, and convert the S1heart sound into morphological data that relates to a rate of change ofpressure within a ventricle of a heart. A housing encloses the controlcircuit. The transducer is located in a region in or on the housing.

According to another embodiment, a method includes using a transducerlocated outside of a heart to detect an S1 heart sound. The S1 heartsound is converted into an electrical signal using the transducer.Morphological data is extracted from the electrical signal. Themorphological data relates to a rate of change of pressure within aventricle of the heart.

According to yet another embodiment, a system includes an implantabledevice and an external system. The implantable device includes atransducer located in or on the implantable device. The transducer isconfigured to convert heart sounds into an electrical signal. A firstcontrol circuit is coupled to the transducer, and is configured toreceive the electrical signal. The implantable device also includes afirst interface circuit for communicating with the external system. Theexternal system includes a second interface circuit for communicatingwith the implantable device. A second control circuit is coupled to thesecond interface circuit. The first and second control circuitscooperate to identify an S1 heart sound, and to generate morphologicaldata from the S1 heart sound. The morphological data relates to a rateof change of pressure in a ventricle of a heart.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts data supporting the notion that the S1 heart soundcontains certain data content related to maximum rate of change of leftventricular pressure.

FIG. 2 depicts a method of analyzing accelerometer data to determine amorphological characteristic related to a hemodynamic metric, accordingto some embodiments of the present invention.

FIG. 3A depicts a chart presenting rate of change in left ventricularpressure (y-axis) versus time (x-axis).

FIG. 3B depicts a chart presenting conditioned accelerometer data(y-axis) versus time (x-axis).

FIG. 4 depicts a signal flow scheme that may be used to implement themethod of FIG. 2, according to some embodiments of the presentinvention.

FIG. 5 depicts an exemplary system for performance of the methods andschemes disclosed herein.

FIG. 6 depicts a method of adjusting parameters that influence aprocess, according to some embodiments of the present invention.

FIG. 7 depicts a method of identifying occurrence of a cardiac event,according to some embodiments of the present invention.

DETAILED DESCRIPTION

During the course of a cardiac cycle, blood flows from the peripheralvenous system to the right atrium. From the right atrium, blood passesthrough the tricuspid valve to the right ventricle. Blood exits theright ventricle, through the pulmonic valve, into the pulmonary artery,and is directed through the lungs, so that the blood may bereoxygenated. Oxygenated blood from the lungs is drawn from thepulmonary vein to the left atrium. From the left atrium, blood passesthough the mitral valve to the left ventricle. Finally, the blood flowsfrom the left ventricle, through the aortic valve, to the peripheralarterial system in order to transfer oxygenated blood to the organs ofthe body.

As the blood circulates and the various valves open and close (as justdescribed), certain heart sounds are produced. The heart sounds occur ina fixed sequence and are respectively referred to as S1, S2, S3 and S4.

The S1 heart sound is caused by acceleration and deceleration of blood,and closure of the mitral and tricuspid valves. The S1 heart soundgenerated during a given cardiac cycle exhibits morphologicalcharacteristics that relate to the maximum rate of change of pressure inthe left ventricle during the given cardiac cycle. The maximum rate ofchange of pressure in the left ventricle is related to, and may be usedas a proxy measurement for, left ventricular contractility. Leftventricular contractility is important, because it indicates thecapacity of the left ventricle to contract, and therefore to circulateblood through the peripheral arterial system.

FIG. 1 depicts data illustrating that the S1 heart sound containscertain data content related to maximum rate of change of leftventricular pressure. FIG. 1 presents a chart having an x-axis and ay-axis. Maximum rate of change of left ventricular pressure for a givencardiac cycle is measured along the x-axis in units of millimeters ofmercury per second (mmHg/s). Median peak-to-peak amplitude exhibited byS1 heart sounds over the past N cardiac cycles is measured along they-axis in units of mG, where N is on the order of 10, for example,between 5 and 25. (One scheme by which “median peak-to-peak amplitude”is determined is discussed below).

To obtain the data presented in FIG. 1, animal testing was performed.During the test, the animal was at rest, and its left ventricularfilling pressure was monitored and determined to be constant. Over aspan of time, a drug known to modify myocardial contractility wasadministered. At intervals, the animal's maximum rate of leftventricular pressure change was measured, and was mated with the medianpeak-to-peak amplitude exhibited by S1 heart sounds over the pastN=approximately 10 (5-25) cardiac cycles, as measured by anaccelerometer located at a point remote from the animal's heart. (Theaccelerometer was located within a cardiac rhythm management deviceimplanted in the animal). Further, the accelerometer data was signalconditioned (discussed below) prior to measurement of the peak-to-peakamplitude. Thus, a given data point on the chart of FIG. 1 is determinedby the maximum rate of left ventricular pressure change during a givencycle, and the median peak-to-peak amplitude exhibited by S1 heartsounds over the past N cardiac cycles.

As can be seen from FIG. 1, the median peak-to-peak amplitude exhibitedover a span of cardiac cycles increases (approximately linearly) withthe maximum rate of left ventricular pressure change. Therefore, bymeasuring the median peak-to-peak amplitude exhibited over a span of Ncardiac cycles, the maximum rate of left ventricular pressure change maybe determined.

Accordingly, FIG. 2 illustrates a method useful in arriving at dataindicative of left ventricular contractility. The example of FIG. 2begins with the reception of raw accelerometer data, as shown inoperation 200. Thereafter, the raw accelerometer data is conditioned(discussed below), for example, to remove noise, respiratory components,and baseline wander (operation 202). The resulting data streamsubstantially represents the sounds emitted by the heart.

This conditioned signal is then processed so as to isolate the S1complex, as shown in operation 204. The result of such a process isdepicted in FIG. 3B. FIG. 3B presents conditioned accelerometer data(y-axis) versus time (x-axis). FIG. 3A presents rate of change in leftventricular pressure (y-axis) versus time (x-axis).

FIG. 3B contains a region identified by a dashed box. The dashed boxidentifies the S1 complex. The process of isolating the S1 complexrefers to identifying a point in time t_(m) at which the S1 complexbegins, and a point in time t_(n) at which the S1 complex ends.

Returning to FIG. 2, after isolation of the S1 complex at operation 204,peak values are extracted at operation 206. For example, the exemplaryisolated S1 complex depicted in FIG. 3B contains a global maxima labeled“Max,” and a global minima labeled “Min.” The minima and maxima are“global” over the span of time between t_(m) and t_(n). The combinedresult of operations 200-206 is that, for each cardiac cycle, theamplitude values at each global maxima and minima exhibited by an S1heart sound are extracted and stored in a manner to preserve theirrelationship to the cardiac cycle from which they were extracted.

Next, in operations 208 and 210, the median minima and the median maximaexhibited over the last N cardiac cycles are found. For example,assuming that the peak values had been extracted from the J^(th) cardiaccycle in a given instance of execution of operation 206, then operations208 and 210 yield the median minima and the median maxima exhibited overcardiac cycles J−N+1 through J.

Finally, in operation 212, the median minima determined in operation 208is subtracted from the median maxima determined in operation 210. Theresult of operation 212 is an example of a “medianpeak-to-peak-amplitude,” as referred to above with reference to FIG. 1.

FIG. 4 depicts a signal flow scheme that may be used to implement themethod of FIG. 2. As can be seen from FIG. 4, raw accelerometer data issupplied to both a bandpass filter 400 and a baseline estimator 402. Thebandpass filter 400 is characterized by upper and lower cutofffrequencies that are set to pass frequency content included in heartsounds. In one example, the lower and upper cutoff frequencies areapproximately 10 Hz and 50 Hz, respectively. The cutoff frequencies arealso set to reject frequency content due to movement of the patient(e.g., walking or muscular twitching), and to pass frequency content ofthe heart sounds.

Typically, the upper and lower cutoff frequencies of a bandpass filter(such as bandpass filter 400) are determined by two sets of poles. Afirst set of poles determines the lower cutoff frequency (e.g.,placement of poles at 10 Hz generates a lower cutoff frequency atapproximately 10 Hz). Similarly, a second set of poles determines theupper cutoff frequency (again, placement of poles at 50 Hz generates anupper cutoff frequency at approximately 50 Hz). In order to yield anarrow passband region, the sets of poles determining the upper andlower cutoff frequencies may be oriented in proximity to one another(i.e., the poles may be “squeezed” together). Unfortunately, such anapproach tends to exhibit a drawback: the filter may ring when driven bysignals with sharp transitions. Since S1 heart sounds tend to exhibitsharp transitions, the bandpass filter 400 may ring if its passband isnarrowed by way of “squeezing” its poles close together.

To address the problem of ringing, the baseline estimator 402 isintroduced. The baseline estimator 402 yields an estimate of alow-frequency baseline upon which the heart sounds in the accelerometerdata are riding. For example, baseline estimator 402 may be anexponentially-weighted historical averaging unit. By subtracting thebaseline estimate yielded by the estimator 402 from the output of thebandpass filter 400 (using the subtracting unit 404), unwantedlow-frequency content is removed from the signal passed by the filter400. This means that the lower cutoff frequency of the filter 400 may berelaxed (i.e., set at a relatively lower frequency), and that thecombined functioning of the baseline estimator 402 and subtraction unit404 will remove low frequency content. By relaxing the lower cutofffrequency of the filter, the frequency space between the sets of polesmay also be broadened, diminishing the likelihood of ringing in thefilter 400.

The combined functioning of passband filter 400, baseline estimator 402,and subtraction unit 404 operate to achieve the effect described withreference to operation 202 in FIG. 2. Thus, the signal yielded bysubtraction unit 404 primarily includes content related to heart sounds.

The signal from the subtraction unit 404 is passed to an S1 isolatorunit 406, which functions to achieve the result described with referenceto operation 204 in FIG. 2. An exemplary method for automaticallyprocessing accelerometer signals to isolate S1, S2, and S3 heart soundsis disclosed in U.S. Pat. No. 5,792,195, issued to Carlson et al. onAug. 11, 1998, which is incorporated by reference herein in itsentirety.

The output of the S1 isolator unit 406 is a set of time-sequenced datarepresenting an S1 heart sound. Such data is passed to a minima/maximaextractor 408 to find the global minima and maxima of the S1 complex, asdescribed with reference to operation 206 in FIG. 2.

Finally, the output of the minima/maxima extractor 408 is passed to amedian calculator 410 to find the median minima and median maximaexhibited by the last N S1 heart sounds, as described with reference tooperations 208 and 210 in FIG. 2.

Returning briefly to FIG. 1, a formula presented therein describes alinear regression of the data contained in the graph. (In the case ofthe data presented in FIG. 1, the formula is y=0.00009x−0.0625). The“reliability” of the linear regression is indicated by R², which is ameasure of the variance explained by the regression model (in the caseof the data presented in the graph of FIG. 1, R²=0.9866). In oneexample, the reliability of the data is improved (and therefore thereliability of the linear regression model, as understood by R² isimproved) by performing the data processing schemes of FIGS. 2-4 uponheart sounds obtained during periods of exhalation; accelerometer dataobtained from heart sounds occurring during periods of inhalation isignored. To distinguish periods of exhalation and inhalation,transthoracic impedance may be examined (transthoracic impedance maydecrease during exhalation, for example).

FIG. 5 depicts an exemplary system useful for detecting S1 heart sounds,and extracting therefrom one or more morphological characteristicsrelated to left ventricular contractility.

In FIG. 5, an exemplary system 500 for detecting and processing heartsounds includes an implantable system 502 and an external system 504.The implantable system 502 and external system 504 are configured tocommunicate via a communications link 506.

The implantable system 502 includes an implantable device 508operatively coupled to a patient's heart by a lead system 512. Thecomponents of the implantable device 508 include an atrial senseamplifier 514, a ventricular sense amplifier 516, an atrial stimulatingcircuit 518, a ventricular stimulating circuit 520, a controller 522, amemory 524, an accelerometer 526, an analog pre-processing circuit 528,an analog-to-digital (A/D) converter 530, and an input/output (I/O)interface 532. The components of implantable device 508 are housedwithin an implantable housing (indicated by the broken lined box in FIG.5), which may be implanted within the patient's chest cavity (e.g., inthe pectoral region) or elsewhere.

The atrial sense amplifier 514, ventricular sense amplifier 516, atrialstimulating circuit 518 and ventricular stimulating circuit 520 areoperatively coupled to lead system 512 via a pair of conductors 534. Thelead system 512 may include an atrial sensing electrode and an atrialstimulating electrode adapted to be disposed in the right atrial chamberof heart and a ventricular sensing electrode and a ventricularstimulating electrode adapted to be disposed in the right ventricularchamber of the heart.

Sensed atrial and ventricular electrical signals generated by thesensing electrodes are applied to the atrial and ventricular senseamplifiers 514 and 516, respectively. Similarly, atrial and ventricularstimulating signals generated by the atrial and ventricular stimulatingcircuits 518 and 520 are applied to the atrial and ventricularstimulating electrodes, respectively. The atrial sense amplifier 514,ventricular sense amplifier 516, atrial stimulating circuit 518, andventricular stimulating circuit 520, are each also operatively coupledto the controller 522.

In other embodiments, other sensing electrode configurations are usedfor internally sensing one or more electrical signals of heart. In oneexample, only one sensing electrode may be used. Alternatively, one ormore electrodes placed within the body but outside of the heart are usedfor sensing cardiac electrical signals. In yet another example, asensing electrode is placed on the implantable housing. In each of theseexamples, the sensing electrodes are operatively coupled to thecontroller 522.

In the embodiment shown in FIG. 5, the sensing electrodes and thestimulating electrodes are disposed in the right side of heart. In otherembodiments, one or more sensing electrode(s) and one or morestimulating electrode(s) are disposed in the left side of the heart (inlieu of being disposed in the right side of the heart, or in addition tosensing electrode(s) and stimulating electrode(s) disposed in the rightside of the heart). The addition of left heart sensing mayadvantageously allow for the resolution of ambiguities due todisassociation of right and left heart conduction.

The controller 522 includes a microcontroller or microprocessor which isconfigured to execute a program stored in a read-only memory (ROM)portion of a memory unit 524, and to read and write data to and from arandom access memory (RAM) portion of the memory unit 524. By executingthe program stored in memory 524, the controller 522 is configured toprocess the atrial and ventricular electrical signals from the atrialand ventricular sense amplifiers 514 and 516, and to provide controlsignals to the atrial and ventricular stimulating circuits 518 and 520.In response, the stimulating circuits 518 and 520 provide stimulatingpulses to heart via atrial and ventricular stimulating electrodes atappropriate times. In other embodiments, the controller 522 may includeother types of control logic elements or circuitry.

The implantable device 508 may be referred to as a dual-chamberpacemaker since pacemaking functions are provided to both atrial andventricular chambers of heart. In another embodiment, the implantablesystem includes a single-chamber pacemaker that senses electricalsignals and provides stimulating pulses to a single chamber of heart. Inyet another embodiment, the implantable system does not provide anystimulation of heart tissues, but includes one or more sensingelectrodes for sensing one or more electrical signals of heart, and forproviding corresponding sensed signals to controller 522. In stillanother embodiment, the implantable system does not provide any sensingelectrodes for sensing any cardiac electrical signals, but is configuredto sense and transmit signals representing heart sounds using a sensorsuch as the accelerometer 526, as described below.

In the remainder of this description, the implantable device 508 isdescribed as a dual-chamber pacemaker for the sake of illustration. Itis to be understood, however, that implantable system 502 need notprovide the stimulation functions described herein, and may provideother functions which are not described herein.

In some embodiments, a minute ventilation output channel and a minuteventilation input channel may be interposed between the controller 522and the ventricular lead. The minute ventilation output channelgenerates a high-frequency, low-voltage signal that is transmitted fromthe ventricular lead (in either unipolar or bipolar mode). The inputchannel receives and conditions the signal. The content of theconditioned signal reveals respiration information.

An accelerometer 526 may be configured to provide sensed signals to theanalog pre-processing circuit 528, which generates an analog outputsignal which is digitized by A/D converter 530. The digitizedaccelerometer signal is received by the controller 522. In theembodiment of FIG. 5, the accelerometer 526 is located within thehousing of implantable device 508. In another embodiment, theaccelerometer 526 is located on the housing of the implantable device.The accelerometer 526 may include, for example, a piezo-electric crystalaccelerometer sensor of the type used by pacemakers to sense the levelof activity of the patient, or may include other types ofaccelerometers. To detect heart sounds, other types of sound-detectingsensors or microphones may also be used, such as a pressure sensor or avibration sensor configured to respond to sounds made by the heart.

In another embodiment, the system 500 includes two or moresound-detecting sensors. In such an embodiment, the plurality of sensedheart sound signals from the plurality of sensors may be individuallytransmitted to external system 504 for display as individual traces, maybe combined (e.g., averaged) by external system 504 before beingdisplayed as a single trace, or may be combined by controller 522 beforebeing transmitted to external system 504 as a single heart sound signal.These sensors may include different types of sensors, sensors that arelocated in different locations, or sensors that generate sensed signalswhich receive different forms of signal processing.

In one embodiment, the accelerometer 526 is configured to generatesensed signals representative of two distinct physical parameters: (1)the level of activity of the patient; and (2) the heart sounds generatedby heart. Accordingly, the analog pre-processing circuit 528 isconfigured to pre-process the sensed signals from the accelerometer 526in a manner which conforms to the signal characteristics of both ofthese physical parameters. For example, if the frequencies of interestfor measuring the patient's level of activity are below 10 Hz, while thefrequencies of interest for detecting heart sounds are between 0.05 Hzand 50 Hz, then analog pre-processing circuit 528 may include a low-passfilter having a cutoff frequency of 50 Hz. The controller 522 may thenperform additional filtering in software, as described above withreference to FIGS. 2-4, for example. Along with filtering, analogpre-processing circuit 528 may perform other processing functionsincluding automatic gain control (AGC) functions.

The analog pre-processing circuit 528 may perform the filtering andbaseline wander removal functions described with reference to operation202 (FIG. 2) and may include modules 400, 402, and 404 (shown in FIG.4). Alternatively, the analog pre-processing circuit 528 may simplyprovide automatic gain control functionality.

In some embodiments, the controller 522 performs one or more of steps204-212 (FIG. 2), and may include one or more of modules 406-410 (FIG.4). In the context of an embodiment in which the analog pre-processingcircuit 528 performs only automatic gain control, the controller mayperform operations 200 and 202 (FIG. 2), and may include modules 400-404(FIG. 4). As discussed below, any operations 200-212 or modules 400-410to be performed digitally by a controller may be performed cooperativelyby the controller 522 within the implantable device 508 and anothercontroller. For example, the controller 522 in the implantable device508 may perform operation 204, and communicate the result to an externalcontroller (contained in a programmer, for example) that performsoperation 206-212. Alternatively, an external controller may perform allof the operations 200 described in FIG. 2.

In another embodiment, the implantable device 508 has two pre-processingchannels for receiving sensed signals from accelerometer 526. In stillanother embodiment, implantable device 508 includes two accelerometers,with one accelerometer configured to generate sensed signalsrepresentative of the level of activity of the patient and the otheraccelerometer configured to generate sensed signals representative ofheart sounds. In these latter two embodiments, any hardware and/orsoftware processing performed on the sensed signals can conform to thespecific characteristics of the respective sensed signals. For example,the analog pre-processing circuit used for the level-of-activity sensedsignals can provide a low-pass filter with a cutoff frequency of 10 Hz,while the analog preprocessing circuit for the heart-sound sensedsignals can provide a band-pass filter with cutoff frequencies of 0.05and 50 Hz. In the latter case, each accelerometer can be selected,located and/or oriented to maximize the detection of the respectivephysical parameter. In yet another embodiment, if the implantable devicedoes not need to sense the level of activity of the patient, theaccelerometer 526 may measure only the sounds made by heart.

The controller 522 is capable of bi-directional communications with anexternal system 504 via an I/O interface 532. In one embodiment, the I/Ointerface 532 communicates using RF signals, which may be understood toinclude inductive coupling. In other embodiments, the I/O interface 532communicates using optical signals, or a combination of RF and opticalsignals (e.g., RF signals for receiving data from the external system504 and optical signals for transmitting data to external system 504, orvice-versa). The controller 522 uses the I/O interface 532 forbi-directional communications with the external system 504 to supportconventional monitoring, diagnostic and configuration pacemakerfunctions. The controller 522 may also use the I/O interface 532 totelemeter data representative of the heart sounds sensed byaccelerometer 526 to the external system 504. In various embodiments,the controller 522 further uses the I/O interface 532 to telemeter datarepresentative of cardiac electrical signals (i.e., electrogram or EGMsignals), which may include data representative of atrial electricalsignals, sensed by the atrial sensing electrode, and/or datarepresentative of ventricular electrical signals, sensed by theventricular sensing electrode. Thus, implantable system 502 is capableof sensing heart sounds, atrial electrical signals and ventricularelectrical signals, and of telemetering data representative of the heartsounds and/or cardiac electrical signals to external system 504. Inother embodiments, the controller 522 telemeters data representative ofcardiac electrical signals which were sensed by other configurations ofinternal cardiac sensing electrodes.

The external system 504 may include an external device 542. The externaldevice 542 may include an external controller 546, an I/O interface 548,user input device(s) 550, and user output device(s) 552. Using the I/Ointerface 548, the external controller 546 is configured forbi-directional communications with the implantable device 508, forreceiving input signals from input device(s) 550, and for applyingcontrol signals to output device(s) 552. The input device(s) 550 includeat least one input device which allows a user (e.g., a physician, nurse,medical technician, etc.) to generate input signals to control theoperation of external device 542, such as at least one user-actuatableswitch, knob, keyboard, pointing device (e.g., mouse), touch-screen,voice-recognition circuit, etc. The output device(s) 552 include atleast one display device (e.g., CRT, flat-panel display, etc.), audiodevice (e.g., speaker, headphone), or other output device whichgenerates user-perceivable outputs (e.g., visual displays, sounds, etc.)in response to control signals. The external controller 546 may beconfigured to receive the data representative of heart sounds, atrialelectrical signals and/or ventricular electrical signals fromimplantable system 502, and to generate control signals that, whenapplied to output device(s) 552, cause the output device(s) to generateoutputs that are representative of the heart sounds, the atrialelectrical signals and/or the ventricular electrical signals.

The external controller 546 may cooperate with the internal controller522 to perform any or all of the steps in FIG. 2. For example, theimplantable device 508 may telemeter conditioned accelerometer data(yielded from operation 202 in FIG. 2) to the external controller 546via the communication link 506. The external controller 546 may performoperations 204-212 upon the telemetered data.

In one embodiment, the system 500 further includes a remote system 554operatively coupled to communicate with the external system 504 viatransmission media 556. The remote system 554 includes one or more userinput device(s) 558, and one or more user output device(s) 560, whichallow a remote user to interact with remote system 554. The transmissionmedia 556 includes, for example, a telephone line, electrical or opticalcable, RF interface, satellite link, local area network (LAN), wide areanetwork (WAN) such as the Internet, etc. The remote system 554cooperates with external system 504 to allow a user located at a remotelocation to perform any of the diagnostic or monitoring functions thatmay be performed by a user located at external system 504. For example,data representative of heart sounds and/or cardiac electrical signalsare communicated by the external system 504 to the remote system 554 viathe transmission media 556 to provide a visual display and/or an audiooutput on the output device(s) 560, thereby allowing a physician at theremote location to aid in the diagnosis of a patient. The system 554 is“remote” in the sense that a user of remote system 554 is not physicallycapable of actuating input device(s) 550 and/or of directly perceivingoutputs generated by output device(s) 552. For example, the system 554may be located in another room, another floor, another building, anothercity or other geographic entity, across a body of water, at anotheraltitude, etc., from the external system 504.

Although not depicted in FIG. 5, the I/O interface 532 may establish acommunication link with a communication device in physical proximity tothe patient. For example, the I/O interface may establish a data linkwith a personal digital assistant, and may upload or download any of thedata mentioned previously or hereafter. The personal digital assistantmay, in turn, establish a link with an access point, so that the linkmay be effectively extended over a network, such as the Internet.

To this point, the disclosure has discussed schemes for detecting aheart sound, generating morphological data from the heart sound, andrelating the morphological data to a hemodynamic metric (FIGS. 1-4). Thedisclosure has also discussed an exemplary device and system forperforming such acts (FIG. 5). The remainder of the disclosure relatesto methods which may make use of the methods discussed with reference toFIG. 1-4, and may make use of the exemplary device or system disclosedwith reference to FIG. 5.

FIG. 6 depicts a method of adjusting parameters that influence aprocess. Any of the acts depicted in FIG. 6 may be performed by anycontroller in the exemplary system of FIG. 5. The method begins withperforming a process known to affect a characteristic of the heart thatinfluences sound, as shown in operation 600. For example, operation 600may include titration of a drug known to influence a heart sound. Aninotrope, for instance, enhances the inotropic state of the heart,resulting in greater myocardial contractility, which influences the S1heart sound (as shown in FIG. 1). Alternatively, operation 600 mayinclude administration of therapy to the heart. For instance, animplantable device (such as the one shown in FIG. 5) may administercardiac resynchronization therapy to the heart. Over time, cardiacresynchronization therapy may make the heart stronger, resulting ingreater myocardial contractility. Again, contractility is known toinfluence the S1 heart sound.

Next, in operation 602, the heart sounds are detected using anaccelerometer that is located at a point that is remote from the heart(e.g., within an implantable device, such as the one depicted in FIG.5). The monitoring operation 602 may include, for example, performingthe acts discussed with reference to FIGS. 2-4. Thus, the medianpeak-to-peak amplitude exhibited by S1 heart sounds may be monitored(after being conditioned, as discussed with reference to FIGS. 2-4) toobtain information regarding the contractility of the left ventricle.

After monitoring the heart sounds in operation 602, it is determinedwhether the cardiac characteristic known to relate to the heart sound isin the desired state. For example, in the context of monitoring S1 heartsounds during administration of cardiac resynchronization therapy, thegoal may be to increase or maximize contractility of the heart byadministration of such therapy. Accordingly, operation 604 may involve adetermination of whether the contractility of the heart has beenmaximized or sufficiently increased. If it is determined that the givencardiac characteristic (e.g., contractility) is, indeed, in the desiredstate, then the process may come to an end, as shown in operation 606.On the other hand, if the desired state has not been reached thencontrol may be passed to operation 608.

Operation 608, is optional. In operation 608, one or more parametersinfluencing execution of the process of operation 600 are changed. Forexample, if operation 600 involved administration of cardiacresynchronization therapy, then one or more resynchronization parametersmay be changed in operation 608. For example, an atrioventricular pacingdelay parameter, biventricular delay parameter, electrode site selectionparameter, etc. may be altered. Execution of operation 608 may beperformed automatically by an implanted cardiac rhythm managementdevice. Alternatively, it may be performed automatically by an externalprocessor (e.g., accelerometer data is telemetered to a programmer orother external device; the accelerometer data is processed according toFIG. 2; the programmer automatically determines how to adjust one ormore parameters on the basis of such processing, and new parameters aretelemetered to the implantable device). Operation 608 may be performedmanually as well (e.g., accelerometer data is telemetered to aprogrammer; the accelerometer data is processed according to FIG. 2; ahealth care professional analyzes the accelerometer data to determinehow to adjust one or more parameters, and new parameters are telemeteredto the implantable device). After execution of operation 608, controlreturns to operation 600, and the controlled process continues.

Operation 608 may be omitted entirely. For example, in the context oftitration, rather than increasing or decreasing the titration rate inoperation 608, the titration rate may remain constant, meaning thatcontrol returns to operation 600 and titration simply continues.

Thus, in sum, the loop defined by operations 600, 602, 604, and 608,functions to monitor and control a process, until a cardiaccharacteristic is observed (by virtue of detection of heart sounds witha remote accelerometer) to be in a desired state.

FIG. 7 depicts a method of identifying an occurrence of a cardiac event.Any of the acts depicted in FIG. 7 may be performed by any controller inthe exemplary system of FIG. 5. The method begins with detecting heartsounds using an accelerometer that is located at a point that is remotefrom the heart (e.g., within an implantable device, such as the onedepicted in FIG. 5), as shown in operation 700. The monitoring operationof operation 700 may include, for example, performing the acts discussedwith reference to FIGS. 2-4. Thus, the median peak-to-peak amplitudeexhibited by S1 heart sounds may be monitored (after being conditioned,as discussed with reference to FIGS. 2-4) to obtain informationregarding the contractility of the left ventricle.

Thereafter, control may be passed to operation 702. Operation 702 isoptional, and may be omitted altogether. In operation 702, theinformation obtained by monitoring of heart sounds in operation 700 iscombined with other information (e.g., exertion level as indicated byrate of respiration and/or motion-related acceleration, etc.). The heartsound information may be combined with any of the information normallymeasured by a cardiac rhythm management device, for example.

Next, in operation 704, the information obtained from operations 700 and702 (if performed) is analyzed to determine whether a cardiac event ofinterest is occurring. For example, the information may be analyzed todetermine whether ischemia or acute heart failure decompensation isbeing exhibited by a patient. If no such event is detected, control isreturned to operation 700, and the monitoring continues. On the otherhand, if such an event is detected, control is passed to operation 706.

Operation 706 is optional. In operation 706, occurrence of the detectedevent is communicated. The communication may occur between an implantedcardiac management device (performing operations 700-706) and aprogrammer, a personal digital assistant, an access point to a network,such as a wireless or wired network, or to a wireless communicationdevice that may forward the message to an access point for communicationto a remote computing system, for example. (The programmer or personaldigital assistant may relay such a message to a remote computing).Alternatively, the communication may occur between the detection routineexecuting on the internal controller and a history logging routineexecuted by the same internal controller. Thus, the occurrence of thedetected event is logged, so that a health care professional may becomeaware of the event, for example, the next time he or she reads the datacontained in the log. After execution of operation 706, control isreturned to operation 700, and monitoring continues.

The method of FIG. 7 may be performed continually or at intervals. Forexample, a patient may be monitored for a given cardiac condition onceper day (e.g., at night) or several times each day, meaning that themethod of FIG. 7 would be executed once per day or several times perday.

Embodiments of the invention may be implemented in one or a combinationof hardware, firmware, and software. Embodiments of the invention mayalso be implemented as instructions stored on a machine-readable medium,which may be read and executed by at least one processor to perform theoperations described herein. A machine-readable medium may include anymechanism for storing or transmitting information in a form readable bya machine (e.g., a computer). For example, a machine-readable medium mayinclude read-only memory (ROM), random-access memory (RAM), magneticdisc storage media, optical storage media, flash-memory devices,electrical, optical, acoustical or other form of propagated signals(e.g., carrier waves, infrared signals, digital signals, etc.), andothers.

The Abstract is provided to comply with 37 C.F.R. Section 1.72(b)requiring an abstract that will allow the reader to ascertain the natureand gist of the technical disclosure. It is submitted with theunderstanding that it will not be used to limit or interpret the scopeor meaning of the claims.

In the foregoing detailed description, various features are occasionallygrouped together in a single embodiment for the purpose of streamliningthe disclosure. This method of disclosure is not to be interpreted asreflecting an intention that the claimed embodiments of the subjectmatter require more features than are expressly recited in each claim.Rather, as the following claims reflect, inventive subject matter liesin less than all features of a single disclosed embodiment. Thus, thefollowing claims are hereby incorporated into the detailed description,with each claim standing on its own as a separate preferred embodiment.

1. A device comprising: a transducer configured to convert anacceleration correlative to a S1 heart sound into an electrical signal;and a control circuit coupled to the transducer, the control circuitconfigured to: receive the electrical signal; identify the accelerationcorrelative to the S1 heart sound; determine a maximum amplitude of theacceleration correlative to the S1 heart sound and a minimum amplitudeof the acceleration correlative to the S1 heart sound; and determine apatient status indicator using an indication of a peak-to-peakdifference between the maximum amplitude of the acceleration correlativeto the S1 heart sound and the minimum amplitude of the accelerationcorrelative to the S1 heart sound; wherein the patient status indicatorindicates a maximum time rate of change of pressure within a ventricleof a heart during a cardiac cycle; and wherein the device is configuredto detect an auxiliary physiological signal in conjunction withdetection of the acceleration correlative to the S1 heart sound, thecontrol circuit configured to use the auxiliary physiological signal indetermining the patient status indicator, the auxiliary physiologicalsignal including a motion-related acceleration.
 2. The device of claim1, wherein the patient status indicator indicates contractility of theheart.
 3. The device of claim 1, wherein the device is configured todetermine an auxiliary indicator using a characteristic of the auxiliaryphysiological signal, and wherein the control circuit is configured to:use the auxiliary indicator in determining the patient status indicator;and provide the patient status indicator for a user or automatedprocess.
 4. The device of claim 1, wherein the device is configured todetect respiration, the control circuit being configured to ignore, forpurposes of determining the patient status indicator, an accelerationcorrelative to a S1 heart sound during a period of inhalation.
 5. Thedevice of claim 1, wherein the transducer is implantable.
 6. The deviceof claim 1, wherein the control circuit is included with an externalsystem, the control circuit being communicatively coupled with thetransducer.
 7. The device of claim 1, further comprising a therapydelivery circuit coupled to the control circuit, the therapy deliverycircuit configured to deliver therapy to the heart in the form ofelectrical impulses.
 8. The device of claim 7, wherein the controlcircuit is configured to adjust, using the patient status indicator, thetherapy delivered by the therapy delivery circuit.
 9. The device ofclaim 7, wherein the device is configured to mate to a lead systemconfigured to deliver electrical impulses to the heart.
 10. The deviceof claim 1, wherein the transducer comprises an accelerometer.
 11. Thedevice of claim 1, wherein the control circuit is configured todetermine the peak-to-peak difference between the maximum amplitude ofthe acceleration correlative to the S1 heart sound and a minimumamplitude of the acceleration correlative to the S1 heart sound by:identifying a time, t_(m), associated with a beginning of theacceleration correlative to the S1 heart sound, and a time, t_(n),associated with an ending of the acceleration correlative to the S1heart sound; identifying, during a span of time between t_(m) and t_(n),a global maximum amplitude of the acceleration correlative to the S1heart sound and a global minimum amplitude of the accelerationcorrelative to the S1 heart sound; determining, for a plurality ofaccelerations correlative to respective S1 heart sounds, a centraltendency of the global maximum amplitudes correlative to the respectiveS1 heart sounds and a central tendency of the global minimum amplitudescorrelative to the respective S1 heart sounds; and subtracting thecentral tendency of the global minimum amplitudes correlative to therespective S1 heart sounds from the central tendency of the globalmaximum amplitudes correlative to the respective S1 heart sounds todetermine the peak-to-peak difference.
 12. A method comprising: using atransducer to detect an acceleration correlative to an S1 heart sound;converting the acceleration correlative to S1 heart sound into anelectrical signal using the transducer; using the electrical signal todetermine a maximum amplitude of the acceleration correlative to the S1heart sound and a minimum amplitude of the acceleration correlative tothe S1 heart sound; determining a patient status indicator using anindication of a peak-to-peak difference between the maximum amplitude ofthe acceleration correlative to the S1 heart sound and the minimumamplitude of the acceleration correlative to the S1 heart sound, whereinthe patient status indicator indicates a maximum time rate of change ofpressure within a ventricle of a heart during a cardiac cycle; anddetecting an auxiliary physiological signal in conjunction withdetection of the acceleration correlative to the S1 heart sound, theauxiliary physiological signal being used in determining the patientstatus indicator, wherein the auxiliary physiological signal includes amotion-related acceleration.
 13. The method of claim 12, wherein thepatient status indicator indicates contractility of the heart.
 14. Themethod of claim 12, comprising: determining an auxiliary indicator usinga characteristic of the auxiliary physiological signal; using theauxiliary indicator in determining the patient status indicator; andproviding the patient status indicator for a user or automated process.15. The method of claim 12, comprising detecting respiration andignoring, for purposes of determining the patient status indicator, anacceleration correlative to a S1 heart sound during a period ofinhalation.
 16. The method of claim 12, comprising storing the maximumamplitude of the acceleration correlative to the S1 heart sound and theminimum amplitude of the acceleration correlative to the S1 heart soundin a datastore, so that a trend regarding the maximum amplitude and theminimum amplitude may be obtained.
 17. The method of claim 12,comprising adjusting, using the patient status indicator, a therapydelivered to the heart.
 18. The method of claim 17, wherein the therapycomprises cardiac resynchronization therapy.
 19. The method of claim 12,comprising adjusting a drug dosage using the patient status indicator.20. The method of claim 12, comprising determining the peak-to-peakdifference between the maximum amplitude of the acceleration correlativeto the S1 heart sound and the minimum amplitude of the accelerationcorrelative to the S1 heart sound by: identifying a time, t_(m),associated with a beginning of the acceleration correlative to the S1heart sound, and a time, t_(n), associated with an ending of theacceleration correlative to the S1 heart sound; identifying, during aspan of time between t_(m) and t_(n), a global maximum amplitude of theacceleration correlative to the S1 heart sound and a global minimumamplitude of the acceleration correlative to the S1 heart sound;determining, for a plurality of accelerations correlative to respectiveS1 heart sounds, a central tendency of the global maximum amplitudescorrelative to the respective S1 heart sounds and a central tendency ofthe global minimum amplitudes correlative to the respective S1 heartsounds; and subtracting the central tendency of the global minimumamplitudes correlative to the respective S1 heart sounds from thecentral tendency of the global maximum amplitudes correlative to therespective S1 heart sounds to determine the peak-to-peak difference.